Frequency-shift-keyed (FSK) modulation is used extensively in data communications in a primary channel of a low speed modem and in a secondary or reverse channel of a high speed modem. Frequency-shift-keying is an abrupt modulation technique that creates a signal having theoretically infinite bandwidth. In frequency division multiplex transmission systems where each channel is allocated a fixed bandwidth the sidebands of an FSK signal can cause adjacent channel interference, therefore some filtering is generally required before the FSK signal can be transmitted. Filtering requirements can be formidable, however, and efforts have been made to shape the spectrum of the FSK signal prior to filtering to ease the filter design requirements and thereby reduce their cost.
In U.S. Pat. No. 3,054,073 issued to K. H. Powers on Sept. 11, 1962, there is disclosed a technique utilizing amplitude and angular-velocity modulation in which the angular-velocity of a carrier wave is modulated by a baseband signal and the amplitude of the angular-velocity modulated carrier is modulated by a signal that is an exponential transform of the baseband signal. Such a technique results in a single sideband angular-velocity modulated signal but is complex and costly to implement. Also, the bandwidth of the resulting single sideband signal may be wider than the original signal.
In U.S. Pat. No. 3,895,316 issued to H. Fein on July 15, 1975, the technique of combining frequency modulation and amplitude modulation without transformation of the baseband signal was utilized. The steps of frequency modulation and suppressed carrier amplitude modulation are used to achieve a "chirping" quality in the resulting audible sound to thereby enhance the discernability of the sound to a listener. This suppressed carrier technique, however, consumes more bandwith than frequency modulation alone.
It is, therefore, an object of this invention to decrease the out-of-band energy content of a frequency modulated signal prior to filtering.
It is another object of this invention to provide spectral shaping in a simple and inexpensive manner.